imgtool: prefer cbor2 over cbor

The cbor module is unmaintained, with the last release in 2016[1]. The
cbor2 module however is under active development and was last released
just last month[2].

As the APIs are identical, we can import cbor2 and if that fails fall
back to cbor.

[1] https://pypi.org/project/cbor/#history
[2] https://pypi.org/project/cbor2/#history

Closes #1189

Signed-off-by: Ross Burton <ross.burton@arm.com>
Change-Id: Iaf2d0df625a200a5cebf72dec4a89877a26194ea
diff --git a/scripts/imgtool.nix b/scripts/imgtool.nix
index 9ac41bc..ea9ba8b 100644
--- a/scripts/imgtool.nix
+++ b/scripts/imgtool.nix
@@ -20,7 +20,7 @@
       python37.pkgs.cryptography
       python37.pkgs.intelhex
       python37.pkgs.setuptools
-      python37.pkgs.cbor
+      python37.pkgs.cbor2
     ]
   );
 in